Amorphous polypropylene is well known in the art. The term amorphous refers to the substantial absence of crystallinity in the polymer; for polypropylene homopolymer this means that the polymer is atactic, without any isotactic segments giving rise to crystallinity, as can be determined by the absence of a melting point and a heat of fusion of less than 10 J/g in DSC. Atactic polymers are defined as having no consistent patterns among chiral sequences [Mark H., Bikales N., Encyclopedia of Polymer Science and Engineering, Volume 9, John Wiley & Sons Inc, 1987, page 800] as can be determined by .sup.13 C NMR [Karger-Kocsis J., Polypropylene, Structure blends and composites, 1. Structure and Morphology, Chapman & Hall, 1995, pages15-19]. It is, for instance, formed as a by-product of the production of isotactic polypropylene using Ziegler-Natta catalysis. This amorphous polypropylene fraction is generally of low molecular weight (wax) with a broad molecular weight distribution, and as such either used in e.g. adhesives applications or discarded as landfill. Although these polymers have been referred to as "amorphous", they often are not completely atactic; rather they contain some isotactic segments.
Amorphous polyolefins, such as the amorphous polypropylene produced using Ziegler-Natta catalysis, have been used in blends with crystalline isotactic polypropylene to improve the flexibility of mechanically strong isotactic polypropylene. Patent document EP 527589 teaches the production of a blend of (a) 20-80 percent by weight of an amorphous polyolefin having a propylene and/or butene-1 component content of 50 percent by weight or more, and (b) 80-20 percent by weight of a crystalline polypropylene. These blends are claimed to be well balanced in mechanical strength and flexibility. However, amorphous polypropylenes produced by Ziegler-Natta catalysis reduce the overall mechanical strength of a blend of amorphous polypropylene and crystalline polypropylene.
Production of high molecular weight amorphous atactic polypropylene with a narrow molecular weight distribution is taught in the art to be attainable by using single site catalysts. Patent documents EP 604917 and EP 604908 teach the synthesis of amorphous polypropylene polymers which are claimed to have interesting elastomeric properties. The characteristics of the polymers, according to EP 604917 are: intrinsic viscosity&gt;1 dl/g, percent (r) minus (-) percent (m)&gt;0 wherein percent (r) is the percent of syndiotactic diads and percent (m) is the percent of isotactic diads, less than 2 percent of the CH.sub.2 groups contained in sequences (CH.sub.2).sub.n with n.gtoreq.2, Bernoullianity index B=1.+-.0.2, and a narrow molecular weight distribution.
Patent documents WO 96/23838, U.S. Pat. No. 5,539,056 and U.S. Pat. No. 5,516,848 teach the production of a blend of an amorphous poly-.alpha.-olefin of molecular weight (Mw) at least about 150,000 (produced using single site catalysis) and a crystalline poly-.varies.-olefin with Mw less than 300,000, (produced using single site catalysis) in which the molecular weight of the amorphous polypropylene is greater than the molecular weight of the crystalline polypropylene. The preferred blends comprise about 10 to about 90 weight percent of amorphous polypropylene. These blends are claimed to exhibit unusual elastomeric properties, namely an improved balance of mechanical strength and rubber recovery properties.
Patent document U.S. Pat. No. 5,483,002 and Patent Document EP 643 100 teach the production of a blend of a semi-crystalline propylene homopolymer having a melting point of 125 to 165.degree. C. and a semi-crystalline propylene homopolymer having a melting point below 130.degree. C. or a non-crystallizing propylene homopolymer having a glass transition temperature which is less than or equal to -10.degree. C. These blends are claimed to have improved mechanical properties, notably impact strength.
Crystalline polypropylene is used in many film or sheet forming processes, such as biaxially oriented PP production, blow molding and thermoforming. In these processes achieving the desired degree of stretching and orientation in the film or sheet is a problem. Stretching must occur between the moment the film or sheet leaves the die as a melt and the time the melt has cooled to a complete solid. Achieving a stable running process, in which the film or sheet production is not limited by breakage or rupturing of the film or sheet is also a problem.